Climate, not transport from “uplands” or “extrabasinal lowlands,” is the cause of drought-tolerant terrestrial organisms in the late Paleozoic fossil record

A long-standing interpretation of the Pennsylvanian “Coal Age” tropical landscape partitions it along an elevational gradient, with wetland, drought-intolerant plants and animals occupying lowland, basinal settings, and increasingly drought-tolerant plants and animals colonizing progressively more remote areas, termed “extrabasinal lowlands” and “uplands”. This model embodies the unspoken, and long-held assumption of a broad, uniform climate across the tropics (warm and wet). The landscape environmental gradient is envisioned as the result of increasing soil drainage with distance from the shoreline and elevation. Consequently, drought-tolerant organisms found in basinal lowlands are, for the most part, assumed to have been transported there from their spatially remote habitats. Here we review a more recent model that incorporates the glacial dynamics of the Pennsylvanian Subperiod in Earth history. Changes in polar ice volume during glacial-interglacial cycles were accompanied by synchronous changes in sea level, atmospheric circulation and attendant climates, and sedimentary dynamics. In tropical basins this resulted in periods of high moisture and low seasonality, producing widespread wetlands with drought-intolerant organisms (high preservation potential), alternating with high-seasonality and variable moisture landscapes populated by organisms tolerant of seasonal dryness and drought (lower preservation potential). Seasonally dry landscapes included wetland forms persisting in wet sub-habitats, or confined to refugia. This latter Climate Model encompasses the full suite of biological, geological, and modeling results better than the unidimensional Upland Model and thus has far greater explanatory potential for the fossil record.